Java按字节截取字符串,中文是两字节

public class Substring {
    public String substring(String input, int n) {
        byte[] bytes = input.getBytes();
        byte[] data = new byte[n]; // 结果


        int x = 0;
        int y = 0;
        while (x < n) {
            int length = Character.toString(input.charAt(y)).getBytes().length;
            if (length > 1) { // 双字节

                if (x >= n - 1) { // 如果是最后一个字节

                    break;
                } else {
                    data[x] = bytes[x];
                    data[x + 1] = bytes[x + 1];
                    x += 2;
                    y++;
                }
            } else {
                data[x] = bytes[x];
                x++;
                y++;
            }
        }
        return new String(data, 0, x);
    }

    public static void main(String[] args) {
        Substring test1 = new Substring();
        System.out.println(test1.substring("你好a,你是?", 3));
        System.out.println(test1.substring("你好a,你是?", 4));
        System.out.println(test1.substring("你好a,你是?", 5));
        System.out.println(test1.substring("你好a,你是?", 6));
        System.out.println(test1.substring("你好a,你是?", 7));
        System.out.println(test1.substring("你好a,你是?", 8));
    }
}